Small sized electronic circuits with high performance have been developed so far. In particular, a multi-functional electronic circuit capable of being mounted in a small space has been provided for cellular phones and computers, for example. Simultaneously, further improvements have been made for effective heat radiation from circuits and for downsizing thereof.
FIG. 5 shows a part of a cellular phone. The cellular phone has a housing 21 in which a circuit board 22 or substrate is received. The circuit board 22 supports a plurality of electronic parts 23, a microprocessor (MPU) 24, a speaker 25 and a microphone 26 mounted thereon. With this cellular phone so constructed, a signal received by an antenna 27 is processed by the microprocessor 24, then transformed into an audio signal by the electronic parts 23, and finally transmitted from the speaker 25 in the form of a voice. On the other hand, voices collected at the microphone 26 are transformed into a corresponding voice signal, then processed by the electronic parts 23 and the microprocessor 24, and finally transmitted from the antenna 27.
Disadvantageously, downsizing of electronic devices requires an elevated performance of heat radiation for the circuit board. This problem may be solved by an enlargement of the circuit, which results in an enlargement of the phone. On the other hand, it is impossible to reduce an area of the circuit board to be less than that of the electronic parts. In addition, currently used circuit board 22 and electronic parts 23 are relatively large in thickness. These circumstances provide a great difficulty of downsizing the cellular phone and result in an increase of its manufacturing cost. To solve such problems, another idea may be proposed to layer a plurality of circuit boards. However, this requires longer wires for electrical connection of electronic parts, which results in a difficulty of its jitter control.
Meanwhile, a double-sided circuit board bearing circuits on opposite surfaces, and a multi-layered circuit board, have been used for compactness and high performance of a circuit substrate of electronic and optical devices. In fact, a high density, multi-layered substrate is employed in a large number of electronic devices.
In a process for manufacturing such a multi-layered substrate, a glass cloth is impregnated with epoxy resin and then dried to produce a substrate material typically called a “prepreg”. Opposite surfaces of the prepreg are covered with a copper film on which a dry film is then laminated. The dry film is exposed to light and then developed to form an etching pattern with which the copper film is etched. Finally, the dry film is removed from the prepreg to result in a double-sided circuit board. Double-sided circuit boards and prepregs are layered alternately and then integrated by heating and pressing into a multi-layered circuit board. Subsequently, holes are defined in the multi-layered circuit board as necessary. Also, an electrically conductive layer is deposited on an inner surface of each hole, thereby causing electrically conductive layers mounted on the circuit board to be electrically connected to each other.
On a surface of the multi-layered circuit board so constructed, a variety of functional parts are mounted to form a specific circuit. Typically, semiconductor chips including an LCR for signal processing, and also one or more packaged logic devices for calculation, are mounted on the surface of the multi-layered circuit board. Further, in order to add other functions required for the electronic and optical devices, a plurality of multi-layered circuit boards can be connected with each other to form a certain module. Furthermore, a certain functional module and/or power circuit may be connected for transformation of an electrical, optical and/or audio signal into necessary information required for a device.
FIG. 9 shows a conventional electronic/optical device using the multi-layered circuit board and the functional modules. As can be seen from the drawing, the device has a multi-layered printed circuit board 231, a flexible board 232, a liquid crystal module 233a, an optical camera module 233b, an input touch panel module 233c, a packaged logic circuit 234, a chip-like functional component 235, a copper wire 236 defined in the multi-layered circuit board, and through-holes 237 each filled with an electrically conductive paste for electrically connecting circuits on respective layers.
In operation of the device using the multi-layered circuit board, optical information is captured and then transformed into a corresponding electrical signal by the optical camera module 233b. This signal is processed into image data by the logic circuit 234 on a bottom surface of the board. Other information inputted through the touch panel module 233c is also transformed into a corresponding electrical signal which is then processed by chip circuit components on the bottom surface of the board, for example. This information is then transformed into respective signal data which is transmitted through the flexible board 232 to another multi-layered circuit board where it is further transformed by logic circuit 234 and functional components 235 such as LCDs. Subsequently, data so transformed is transmitted through the flexible board 232 to the liquid crystal module 233a where it is transformed into a corresponding optical signal and then displayed in the form of an image.
As described above, functional devices and components mounted on opposite surfaces of the multi-layered circuit board are electrically connected by various wires mounted within and/or between layers. Also, copper wires 236 (circuit pattern) of respective layers and an electrically conductive paste 237, filled into through-holes for connection of the copper wires 236, are electrically connected with each other to form a three-dimensional circuit.
However, in this three dimensional circuit, the functional components are mounted only on each surface of the layers. This results in various difficulties with regard to shortening of the wires between the components and/or modules, and requires a flexible circuit board, for example, for electrical connection of the layers, which may cause an adverse affect such as loss and/or noise during transmission of high frequency signals. Also, chip components and packaged functional devices are connected to the board by soldering, for example, which makes it difficult to increase a performance of an overall circuit due to possible inaccuracies of the components and their mounting. This further provides great difficulty with regard to manufacturing of a high-frequency and high-speed digital device, and accordingly, with regard to downsizing and high-functionalization of the device.
FIG. 13 shows a conventional flex-rigid circuit board 301. The board 301 has an unfoldable rigid portion 302 and a foldable flexible portion 303. The flexible portion 303 supports a multi-layered circuit board 304 bearing a conductive circuit 305. The conductive circuit 305 is covered by a protective film 306 bonded thereon. In the rigid portion 302, a rigid circuit board 308 is layered on and bonded to the multi-layered circuit board 304 using a bonding sheet and/or prepreg 307. Also, in the flexible portion 303 where the multi-layered circuit board 304 is exposed, the bonding sheet and/or prepreg 307 and an associated part of the rigid circuit board 308 are removed therefrom. A plurality of electronic components 309 are mounted on the rigid circuit board 308 of the rigid portion 302. These components 309 are electrically connected by through holes 310 running through the multi-layered circuit board 304 and the rigid circuit board 308.
As described above, the conventional flex-grid circuit board 301 lacks flexibility in a region including the multi-layered circuit board 304, which results in great difficulties with regard to configuration of the circuit board, and accordingly, with regard to its installation into a housing with curved portions. Also, the multi-layered circuit board 304 has a certain thickness which prevents shortening of wires in its direction and fails to meet high-frequency requirements. Further, a number of through-holes should be arranged in a complex manner for connection of layered circuits, which provides various restrictions on a circuit design and downsizing of the circuit board, for example, and increases manufacturing costs of the circuit board.
Additionally, according to this conventional multi-layered circuit board, each layer is fully bonded to an adjacent layer. Also, only after completion of manufacturing of the circuit board, is a test performed to confirm whether each layer operates in an expected manner. This is because it can be thought that the multi-layered circuit board works well only after all the layers have been fabricated and then connected with each other. This means that no operational test could be performed until completion of manufacturing of the multi-layered circuit board, which makes it difficult to determine or remove defective products during a process of manufacturing, which results in a decrease of a yield rate and makes products costly.